Senior members of the Ulster Unionist Party have voted to withdraw from Northern Ireland's power-sharing government over claims the Provisional IRA (PIRA) still exists.

UUP leader Mike Nesbitt proposed the exit in response to a police assessment that structures of the supposedly defunct paramilitary organisation remain in place and some of its members were involved in a recent Belfast murder.

The UUP's ruling executive approved Mr Nesbitt's recommendation at a meeting in an east Belfast hotel.

Mr Nesbitt has said the revelations about the IRA have shattered trust in Sinn Fein and the UUP can no longer work in coalition with the republican party.

The vote involving around 90 members of the party's ruling body was unanimous in favour of a Stormont Executive walkout.

"The Ulster Unionist Party will be leaving the Northern Ireland Executive next week," Mr Nesbitt announced after the 90 minute meeting at the Park Avenue Hotel.

"This decision was unanimous."

Mr Nesbitt said Danny Kennedy, the UUP's one minister in a five-party administration comprising 13 ministers and two junior ministers, will formally resign next week.

The party said it will now form an "opposition" in the Assembly - even though the power-sharing structures do not afford recognition for an official opposition to the mandatory coalition government.

The exit by one of the three minor partners in the administration will not in itself trigger the collapse of the Executive but it will throw its future into serious doubt.

The UUP decision will heap pressure on the Democratic Unionists, one of the two major Executive partners, to follow suit - a move that would bring down power-sharing.

The DUP has so far insisted Sinn Fein should be the party leaving the Executive, not unionists. But the largest unionist party has made clear it will walk away if action is not taken to punish Sinn Fein.

The furore was sparked after Police Service of Northern Ireland (PSNI) chief constable George Hamilton said the PIRA still exists and some members were involved in the murder earlier this month of Kevin McGuigan, 53, in co-operation with a group styling itself Action Against Drugs.

Mr Hamilton has, however, said the PIRA is not engaged in terrorism - instead pursuing peaceful, political republicanism - and that there is no evidence the McGuigan killing was sanctioned by the IRA leadership.

Mr McGuigan was suspected by some in the republican movement of involvement in the murder of former IRA leader Gerard "Jock" Davison in Belfast three months ago.

Police believe his killing was a revenge attack by Mr Davison's republican associates.

Mr Nesbitt claimed Sinn Fein's continued insistence that the IRA no longer exists means his party cannot trust what they say.

He said the UUP would now offer an "alternative" to the current Stormont government.

"Sinn Fein's position with regard to the murder of Mr McGuigan has broken all faith and trust," he said.

"Until that trust is restored and we find mechanisms to verify trust we cannot work in government with Sinn Fein."

Sinn Fein, which maintains the IRA has "gone away", has accused the UUP of contriving a crisis in a bid to outflank the DUP ahead of next year's Assembly poll.

The Ulster Unionists have also found themselves branded hypocrites by critics who have pointed to the fact the party co-operated with political representatives of loyalist paramilitaries as part of a pan unionist/loyalist approach to a parading dispute last year.

It is almost 20 years since the Provisional IRA's last ceasefire and a decade on from the supposed decommissioning of its weapons.